A wide variety of apparatus suitable for generating an analog signal, e.g., a DC voltage, proportional to either the frequency or period of a signal of unknown frequency/period have been proposed. Many such apparatus charge a capacitor for a measurement interval determined by the period of the unknown signal and, thus, depend upon the fact that a capacitor charges in accordance with the mathematical expression i=C dv/dt. As long as the value of the capacitor (C) is known, and the current into the capacitor (i) and the change in voltage across the capacitor (dv) are calibrated (or known), the time change (dt) can be easily determined. Depending upon the linearity of such apparatus, dt is directly related to the period and inversely related to the frequency of the unknown signal.
Apparatus that utilize a capacitor charge technique to determine the period or frequency of an unknown signal (commonly called period or frequency converters) can be divided into two general groups. One group comprises circuits that continuously sample the unknown signal and produce an output that is a running average of the capacitor charge. One of the problems with these circuits is that they require heavy filtering of the continuous output (which contains a large AC voltage component as well as the desired DC voltage). Because of the heavy filtering required, these techniques tend to be useful only at frequencies above a few hundred Hz (periods below a few thousandths of a second). Below roughly 200 Hz, i.e., in the low frequency range, the settling time of the filter required becomes very slow (of the order of several seconds). As a result, for low frequency (long period) signals, these circuits are only useful if the frequency of the signal changes slowly. Examples of circuits utilizing this approach are described in U.S. Pat. No. 3,416,082 (Clerc), 3,040,983 (Bigelow), and 3,323,049 (Hanken).
The second group of circuits uses some form of sample and hold technique to store the output. These circuits convert the period of the unknown signal to a voltage (analog signal) one cycle at a time and update the sampled and held output at the end of each conversion cycle. Thus, the sampled and held signal is proportional to the period of the unknown signal. Because the passage of a single cycle is theoretically adequate to provide a valid output voltage, these circuits are useful at much lower frequencies than the first group of circuits discussed above. Examples of circuits that utilize a sample and hold technique are described in U.S. Pat. No. 3,535,658 (Webb) and 3,743,940 (Yamagata).
While the present invention falls in the latter group described above, i.e., the sample and hold group, it provides a substantially improved (in accuracy) output. Specifically, previously developed apparatus using the techniques described above are accurate to approximately plus or minus one percent (at best). This accuracy limitation occurs because both of these prior art techniques depend on the time and temperature stbility of a capacitor, a current source and various other circuitry. Because of the lack of capacitors and other circuit elements having extremely good time and temperature stability, plus good linearity, the plus or minus one percent in accuracy noted above occurs. More specifically, while capacitors can be made with low temperature coefficients, such capacitors tend to have poor dielectric properties. The poor dielectric properties result in such capacitors making nonlinear ramp generators, whereby electronic conversion circuits using such capacitors are inaccurate. Further, while resistors having very low drift can be made, ramp generators utilizing such resistors are highly nonlinear. While field effect transistors or other circuit elements can be added to such generators to linearize their outputs, such elements have a strong temperature dependence. In other words, linearity improvements are made at the cost of a reduction in temperature and time stability. In essence, this statement summarizes the foregoing discussion. That is, when components (capacitors, resistors, etc.) are chosen to improve linearity, prior art period and frequency converters become less time and temperature stable and vice versa. Yet there remains a demand for a precision low frequency period to voltage converter that is both linear and stable with respect to time and temperature. For example, a demand exists for such a converter in the measurement of the rotational period of rotating machinery. A demand also exists in the data logging field where much of the equipment is voltage oriented, i.e., the data logging equipment is adapted to receive information in analog (voltage) form.
Therefore, it is an object of this invention to provide a new and improved apparatus for determining the period of an unknown signal.
It is a further object of this invention to provide a period-to-voltage converter.
It is yet another object of this invention to provide a period-to-voltage converter that is both linear and stable, with respect to both time and temperature.
While various devices have been proposed for determining the ratio of the periods (or frequencies) of a pair of unknown signals, for various reasons, these proposals have been unsatisfactory. Many of them use period-to-voltage converters of the types discussed above and, thus, have the disadvantages discussed above. In addition, most prior art ratio-to-voltage converters include independent channels for converting each signal separately. In addition to increasing the inaccuracy of the result because the time and temperature stability of the channels usually fluctuate differently, the inclusion of two converting channels makes the systems more complex than desirable.
Therefore, it is a further object of this invention to provide a new and improved apparatus for determining the ratio of the periods of a pair of unknown signals.
It is another object of this invention to provide a new and improved ratio (period)-to-voltage converter.
It is yet another object of this invention to provide a ratio (period)-to-voltage converter that is both linear and stable, with respect to both time and temperature.
It is a composite object of this invention to provide a single apparatus that can be used to determine either the period of an unknown signal or the ratio between the periods of a pair of unknown signals.